Catalytic oxidation. III. The mechanism of ethylene and propylene oxidation over supported rhodium and ruthenium

The kinetics of ethylene and propylene oxidation over Rh and Ru supported on low area α-alumina were studied and found similar to those reported earlier for the same metals supported on high area silica. The selectivities for product formation were also analogous. The most selective reaction was oxidation of C₃H₆ over Rh at 200 °, where substantial amounts of acrolein and acetone were formed. Substitution of deuterium for hydrogen in the olefin methyl-group reduced the rates of these reactions significantly, showing that abstraction of H from this position was involved in the rate-determining steps. The acetones formed from labeled propylenes contained deuterium in all possible positions, but exchange into the product acroleins was minor and confined mainly to the 2-position. The acrolein from CD₃CHCH₂ and CH₃CHCD₂ contained nearly equal amounts of the d₁ and d₂ species, but the unreacted propylene was unisomerized. The absolute values for the deuterium contents showed that the bulk of the acrolein was formed via a symmetric allyl-type intermediate, but that up to 26% of the reaction proceeded with the direct conversion of the methyl group of the olefin into the aldehyde group. The data also showed that the isotope effect for the removal of the second H (or D) from the olefin is minimal. Possible mechanisms to explain these findings are presented.